2.Viruses use up valuable system resources, slowing down the system as they do so - okay, Windows does that.

3.Viruses will, from time to time, trash your hard disk - okay, Windows does that too.

4.Viruses are usually carried, unknown to the user, along with valuable programs and systems. - Sigh.. Windows does that, too.

5.Viruses will occasionally make the user suspect their system is too slow (see 2) and the user will buy new hardware. - Yup, Windows does that, too.

Until now it seems Windows is a virus but there are fundamental differences: Viruses are well supported by their authors, are running on most systems, their program code is fast, compact and efficient and they tend to become more sophisticated as they mature.